Each species has developed its own strategy for implantation. Using a cross- species heterologous microarray we identified C-type natriuretic peptide (CNP) as a possible implantation signaling molecule in hamsters which exhibit progesterone- dependent implantation similar to rabbits, pigs, monkeys and most likely in humans. CNP is the third member of the natriuretic peptide (NP) family which also includes atrial- and brain-NPs (ANP and BNP). All NPs act via two types of guanylyl cyclase (GC) receptors, GC-A and GC-B. CNP has more preference for GC-B, while ANP and BNP prefer GC-A. Our preliminary results show that while CNP signaling predominates at the implantation site of hamsters, both ANP/BNP and CNP signalings are active at the implantation sites of both mice and hamsters. These observations together with uterine relaxation properties of CNP and ANP, trophoblast outgrowth by BNP, induction of implantation by BNP, and infertility in GC-B null females suggest that NP signaling strongly influences the implantation process. Thus, we formulated a working hypothesis that NP ligand-receptor signaling influences several biologically and clinically important aspects of implantation: 1) muscular tone of the receptive uterus, 2) blastocyst-uterine cross talk prior to implantation, 3) trophoblast attachment, outgrowth and invasion, and 4) uterine stromal cell decidualization. Therefore, our Specific Aims are to study in hamsters: 1) influence of the blastocyst on the uterus prior to and at the time of implantation;2) functions of NPs in regulation of uterine contractility prior to and during the time of implantation;and 3) the role of NP ligand-receptor signaling in initiation of implantation and decidualization. We will use multiple experimental approaches including qPCR, Northern and in situ hybridization, immunohistochemistry, in vivo adenoviral vector-driven gene inhibition, in vitro uterine contraction studies, and others to accomplish our goals. Deciphering the regulatory events required for implantation will provide insight into the potential causes of defects in implantation and infertility in women. Thus, these studies in hamsters that show progesterone-dependent implantation may provide useful information in the development of diagnostic and therapeutic tools that can be used in detection, prevention and treatment of female reproductive disorders, and improved technologies for assisted reproduction and contraception.